This invention relates to electrophotographic apparatus for forming an image, for example, for obtaining a copy of an original pictorial image. More particularly, the present invention relates to improvements in such apparatus which use an amorphous silicon photoreceptor such that generation of ozone and nitrogen oxides can be prevented, the active life of the photoreceptor can be increased and an image with high contrast can be obtained even with light of low intensity.
In copiers and other electrophotographic image forming apparatus, it has been known to make use of a photoreceptor with photoconductive characteristics whereby electrical resistance is reduced on areas exposed to light with such an apparatus, the photoreceptor is preliminarily charged electrostatically and if its surface is exposed to light reflected, for example, from an original document to be copied, electroconductivity results in areas where light intensity is high such as those corresponding to a white, or unprinted, area on the original document. Electrical charges in these areas are thereby cancelled and there results an electrostatic latent image. If toner is electrostatically attached to this electrostatic latent image and if the attached toner is transferred to a sheet of transfer paper, a visible image of the original document can be obtained. After the transfer process, the photoreceptor is cleaned by a cleaner and a similar series of processes is repeated to form another image. For charging the photoreceptor electrostatically, use is commonly made of a corona discharger. Methods of charging it by means of a charged brush or a charged roller have been proposed but since the electrical resistance of the photoreceptor is low, it is nearly impossible to uniformly charge it by such means and the use of a corona discharge is currently the only practically accepted method.
Speed of the image formation process described above is largely dependent upon the step of exposing the photoreceptor to light reflected from the original document. It is because a certain minimum amount of light energy must be absorbed by the photoreceptor before its surface becomes partly conductive but since there is a limit to which the light intensity can be increased, the time of light exposure must be made long enough for the photoreceptor to absorb the necessary amount of light energy. Since the original document must be exposed to light each time a copy is to be made, it is time-consuming by a conventional image formation method to obtain a large number of copies.
Recently, Japanese Patent Publication Tokkai 58-70238 disclosed a new image formation method by using a photosensitive body having the characteristic of increasing resistance when exposed to light. In the case of amorphous silicon, such characteristic of increasing resistance is known as the Stebler-Ronsky effect and a prior art image forming apparatus and method by using this effect is schematically shown in FIG. 1 wherein numeral 91 indicates a photoreceptor having a photosensitive layer 81 formed on an electroconductive drum 82. Numeral 92 symbolically indicates an optical system for causing a light beam with sufficient energy to fall upon the photoreceptor 91 and to thereby change its electrical resistance locally. An image of an original document to be copied (not shown), for example, is thereby formed on the surface of the photoreceptor 91 by the change of its resistance. Such an image will be referred to as a resistance image in order to distinguish it from images of other latent or visible types.
With reference still to FIG. 1, the photoreceptor 91 keeps rotating in the direction of the arrow and numeral 93 indicates a corona discharger for charging the photoreceptor surface by a corona discharge. When there is a corona discharge, charge is retained only where light has been made incident by the optical system 92 and the photoreceptor surface has increased resistance, thereby converting the aforementioned resistance image into an electrostatic image. Thereafter, toner is attached to this latent electrostatic image by a developing device 94 as done in the conventional electrophotographic process and a visible toner image is formed on the photoreceptor 91. Thereafter, this toner image is transferred onto a transfer medium 83 such as a sheet of transfer paper by means of a transfer charger 95 in a manner well known in electrophotography. Since the resistance image still remains on the photoreceptor surface after the transfer of toner image as described above, another electrostatic latent image can be formed thereon if static charge is again attached thereonto by another corona discharge by the charger 93. In other words, another copy of the same original can be produced without exposing the original to light again. In general, any number of copies of the same original can be obtained after exposing the original to light once. This has the favorable consequence of speeding up the process of obtaining many copies of a single original. In FIG. 1, numeral 96 indicates a charge removing discharger for removing the charge on the photoreceptor surface and numeral 97 indicates a toner cleaner for removing residual toner remaining unused on the aforementioned photosensitive layer 81. The resistance image is erased, if so desired, by heating the photoreceptor 91 to about 150.degree.-200.degree. C. by means of a heater 98 and an infrared lamp 99.
Conventional image formation methods and apparatus have many disadvantages and drawbacks. Firstly, as described above, some toner inevitably remains on the photoreceptor surface after the toner image is transferred and a cleaning process must be included to remove the residual toner. Since the toner transfer efficiency is generally about 70-80%, this means that as much as 20-30% of the developed toner is not utilized, collected in the cleaning process and eventually discarded. From the economical point of view, too, it is very wasteful. If the cleaning process is dispensed with, however, a so-called ghost may appear in the next image forming cycle due to the toner remaining on the photoreceptor surface. Moreover, if the photoreceptor is operated continuously over a long period of time, the residual toner may melt as a result of stress thereon such as heat, forming a thin film of toner on the photosensitive layer and thereby adversely affecting the quality of produced image with reduced image density, etc. In order to avoid such undesirable consequence, a maintenance work will be frequently required for removing the thin toner film on the photosensitive layer. A cleaning blade or a fur brush is frequently used for removing the residual toner but their use tends to injure or disfigure the photoreceptor surface.
Secondly, the corona discharge by the charger not only serves to uniformly charge the photoreceptor as desired but also causes ozone and nitrogen oxides to be formed from oxygen and nitrogen in air. Such ozone and nitrogen oxides, if they come into contact with the photoreceptor surface, tend to oxidize it and/or absorb moisture in air, thereby forming a thin oxide film on the photoreceptor surface. Since such an oxide film has low electrical resistance and is hardly capable of holding electrical charges, it has the undesirable effect of disturbing the electrostatic latent image formed on the photoreceptor surface and hence producing a foggy image. Moreover, generation of ozone itself is undesirable from the point of view of environmental pollution.
Thirdly, there has been a problem of contrast. Conventional photoreceptors for forming a resistance image are structured as shown in FIG. 2 with an amorphous silicon layer 81 formed on the surface of an electroconductive drum 82 made of Al or the like. Since the ratio by which such a conventional photoreceptor changes its electrical resistance by exposure to light is relatively small, a large amount of light energy is required if a sharp contrast is desired between light and dark areas in the image. The time-rate of change in resistance is typically as shown in FIG. 3. Thus, the exposure to light must be continued at least for a time duration indicated by the double-headed arrow in FIG. 3 in order to obtain a reasonably contrasty image.